In earlier prior art, it was customary to use load cell systems for determining the total weight of loads supported by a vehicle, but such systems require special kinds of installation, which is not always convenient. Early strain gauge-type systems eased installation problems, but four strain gauges were initially required for each support member, (e.g. axle, trailer tongue, or the like) to produce the required signals.
In a subsequent advancement of the prior art, U.S. Pat. No. 3,899,924 (Klein), assigned to the assignee of the present invention, disclosed a strain gauge load measuring system requiring only two strain gauges per load support member. This prior art system employed, for a four axle vehicle, eight strain gauges arranged in a pair of bridges energized from independent square wave voltage sources. The output terminals of such bridges were connected serially between ground and a bucking voltage supply energized by a third independent square wave voltage supply. The resulting difference voltage, a function of the net load weight carried by the vehicle, was amplified and then applied to a phase detector requiring a fourth alternating voltage supply and a suitably adjusted potentiometer. A fifth alternating voltage supply drove a DC supply for various DC actuated components in the circuit. The five alternating supplies above-mentioned were provided by five secondary windings of a toroid transformer.
In general, the latter prior art system has performed satisfactorily. However, such prior system is relatively complex and costly to manufacture, for example in its requirement for plural AC supplies at least some of which are of square wave form. Further, care must be taken in the conjunction and location of the toroid transformer to avoid radiation of magnetic energy therefrom in a manner to interfere with proper operation of other circuit components.
Accordingly, the ojects of the present invention include provision of:
1. A multiplexed strain gauge bridge system, particularly for measuring the load on a load support member by application of strain gauges or the like thereto, and particularly adaptable to measuring the net load carried by a vehicle (such as a truck, wagon, trailer or the like) by monitoring the output of strain gauges carried by the vehicle axles or other load support members thereof.
2. A system, as aforesaid, which avoids the need for multiple alternating voltage particularly multiple square wave supplies, avoids AC bridge circuitry, avoids the need for more than one bridge even to monitor the load on four separate load support members of a given vehicle, and further avoids the need for a bucking voltage supply.
3. A system, as aforesaid, which provides a DC voltage bridge output of low amplitude dependent on the total load carried by the load support members and permits high gain amplification of such small DC bridge output signal to provide a DC output suitable for driving indicating means such as a meter, or the like, while avoiding introduction of temperature dependent offsets in the amplified DC signal despite substantial temperature variation during operation of the system.
4. A system, as aforesaid, which avoids the need for toroid transformers, and avoids incorporation of sources of significant magnetic leakage flux or other similar interference as may introduce nonlinearities in the operation of circuit components.
5. A system, as aforesaid, which provides for time division multiplexing of the DC bridge output signal prior to high gain AC amplification and subsequently provides for demodulation of the resulting amplified square wave signal as a DC signal which is a much amplified version of the DC bridge output signal, and which introduction of significant DC offsets is avoided during multiplexing, amplification and demodulating.
6. A system, as aforesaid, which eliminates the need for a complex phase detection circuit as such, particularly one requiring an AC supply.
7. A system, as aforesaid, of simplified circuitry and reduced current drain.
Other objects and purposes of this invention will be apparent to persons acquainted with apparatus of this general type upon reading the following specification and inspecting the accompanying drawings.